Gas mixture control in a gas discharge light source

ABSTRACT

A gas discharge light source includes a gas discharge system that includes one or more gas discharge chambers. Each of the gas discharge chambers in the gas discharge system is filled with a respective gas mixture. For each gas discharge chamber, a pulsed energy is supplied to the respective gas mixture by activating its associated energy source to thereby produce a pulsed amplified light beam from the gas discharge chamber. One or more properties of the gas discharge system are determined. A gas maintenance scheme is selected from among a plurality of possible schemes based on the determined one or more properties of the gas discharge system. The selected gas maintenance scheme is applied to the gas discharge system. A gas maintenance scheme includes one or more parameters related to adding one or more supplemental gas mixtures to the gas discharge chambers of the gas discharge system.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.62/276,522, filed Jan. 8, 2016 and titled GAS MIXTURE CONTROL IN A GASDISCHARGE LIGHT SOURCE, which is incorporated herein by reference in itsentirety.

TECHNICAL FIELD

The disclosed subject matter relates to controlling a gas mixture in agas discharge light source that produces a pulsed light beam.

BACKGROUND

One type of gas discharge light source used in photolithography is knownas an excimer light source or laser. An excimer laser typically uses acombination of a noble gas, such as argon, krypton, or xenon, and areactive such as fluorine or chlorine. The excimer laser derives itsname from the fact that under the appropriate condition of electricalstimulation (energy supplied) and high pressure (of the gas mixture), apseudo-molecule called an excimer is created, which only exists in anenergized state and gives rise to amplified light in the ultravioletrange.

Excimer light sources are used in photolithography machines. The excimerlight source produces deep ultraviolet (DUV) light. An excimer lightsource can be built using a single gas discharge chamber or using aplurality of gas discharge chambers.

SUMMARY

In some general aspects, a gas discharge light source includes a gasdischarge system that includes one or more gas discharge chambers, eachgas discharge chamber housing an energy source. The gas discharge lightsource is operated. Each of the gas discharge chambers in the gasdischarge system is filled with a respective gas mixture. For each gasdischarge chamber, a pulsed energy is supplied to the respective gasmixture by activating its energy source to thereby produce a pulsedamplified light beam from the gas discharge chamber. One or moreproperties of the gas discharge system are determined. A gas maintenancescheme is selected from among a plurality of possible schemes based onthe determined one or more properties of the gas discharge system. Theselected gas maintenance scheme is applied to the gas discharge system.A gas maintenance scheme includes one or more parameters related toadding one or more supplemental gas mixtures to the gas dischargechambers of the gas discharge system.

Implementations can include one or more of the following features. Forexample, the one or more properties of the gas discharge system can bedetermined by determining one or more properties of each of the gasdischarge chambers in the gas discharge system.

The selected gas maintenance scheme can be applied to the gas dischargesystem by applying the selected gas maintenance scheme to each of thegas discharge chambers of the gas discharge system.

The gas discharge system can include two gas discharge chambers.

The gas discharge chamber can be filled with the respective gas mixtureby filling the gas discharge chamber with a mixture of a gain medium anda buffer gas. The gas discharge chamber can be filled with the mixtureof the gain medium and the buffer gas by filling the gas dischargechamber with a gain medium that includes a noble gas and a halogen, anda buffer gas that includes an inert gas. The noble gas can includeargon, krypton, or xenon; the halogen can include fluorine; and theinert gas can include helium or neon.

The pulsed energy can be supplied to a respective gas mixture byactivating its energy source by applying a pulsed voltage to a pair ofelectrodes within the gas discharge chamber so that an electricalstimulation is applied to a halogen within the gas mixture.

The one or more properties of the gas discharge system can be determinedby determining an age of at least one of the gas discharge chambers ofthe gas discharge system based on one or more of: how many times thatgas discharge chamber has been filled with the gas mixture and how oftenthe energy source of that gas discharge chamber has been activated.

The one or more properties of the gas discharge system can bedetermined, the gas maintenance scheme can be selected, and the selectedgas maintenance scheme can be applied to the gas discharge system whilepulsed energy is being supplied to the gas mixture of the one or moregas discharge chambers. The one or more properties of the gas dischargesystem can be determined, the gas maintenance scheme can be selected,and the selected gas maintenance scheme can be applied to the gasdischarge system while pulsed energy is not being supplied to the gasmixture of any of the gas discharge chambers.

The gas maintenance scheme can be selected from among the plurality ofpossible gas maintenance schemes by selecting a standard gas injectionscheme if it is determined that an age of at least one of the gasdischarge chambers is in a first range, and the selected gas injectionscheme can be applied to the gas discharge system by pumping at least afirst amount of a buffer gas into the gas mixture of the at least onegas discharge chamber. The gas maintenance scheme can be selected fromamong the plurality of possible gas maintenance schemes by selecting aconservation gas injection scheme if it is determined that an age of theat least one gas discharge chamber is in a second range. The selectedconservation gas injection scheme can be applied to the gas dischargesystem by pumping a second amount of the buffer gas into the gas mixtureof the at least one gas discharge chamber, the second amount being lowerthan the first amount. The gas maintenance scheme can be selected fromamong the plurality of possible gas maintenance schemes by selectinganother conservation gas injection scheme if it is determined that anage of the at least one gas discharge chamber is in a third range. Theselected gas injection scheme can be applied to the gas discharge systemby pumping a third amount of the buffer gas into the gas mixture of theat least one gas discharge chamber, the third amount being lower thanthe first amount but greater than the second amount.

The first range can be a value less than or equal to a lower value, thesecond range can be a value greater than the lower value, and the thirdrange can be a value that is greater than an upper value. The firstrange can be a first value of the age, the second range can be a secondvalue of the age, and the third range can be a third value of the age.

The first range can be a first value of the age and the second range canbe a second value of the age. The second range can be distinct from thefirst range.

The gas maintenance scheme can be selected from among the plurality ofpossible gas maintenance schemes by selecting a standard gas injectionscheme if it is determined that an age of at least one of the gasdischarge chambers is in a first range. The selected gas injectionscheme can be applied to the gas discharge system by performing aninjection of a buffer gas into the gas mixture of the at least one gasdischarge chamber at a first temporal frequency. And, gas maintenancescheme can be selected from among the plurality of possible gasmaintenance schemes by selecting a conservation gas injection scheme ifit is determined that the age of the at least one gas discharge chamberis in a second range. The selected conservation gas injection scheme canbe applied to the gas discharge system by performing an injection of thebuffer gas into the gas mixture of the at least one gas dischargechamber at a second temporal frequency that is different from the firsttemporal frequency.

The second temporal frequency can be less than the first temporalfrequency. An injection of the buffer gas into the gas mixture of the atleast one gas discharge chamber can be performed by also injecting oneor more components of the gain medium into the gas mixture of the atleast one gas discharge chamber.

The method can also include monitoring one or more operatingcharacteristics of the gas discharge light source; determining whetherany of the one or more monitored operating characteristics will be outof an acceptable range at a future time; and, if it is determined thatany of the one or more monitored operating characteristics will be outof an acceptable range at a future time, then: selecting a restore gasmaintenance scheme and applying the selected restore gas maintenancescheme to the gas discharge system.

The selected restore gas maintenance scheme can be applied to the gasdischarge system by applying one or more of a restore injection schemeand a refill scheme to the gas discharge system. The selected restoreinjection scheme can be applied to the gas discharge system byperforming an injection of a buffer gas into the gas mixture of at leastone of the gas discharge chambers that increases a relative amount ofthe buffer gas in the gas mixture of the at least one gas dischargechamber. The injection of the buffer gas into the gas mixture can beperformed by one or more of altering a temporal frequency at which theinjection is performed and pumping a different amount of buffer gas intothe gas mixture than was pumped before it was determined that any of theone or more monitored operating characteristics will be out of anacceptable range.

The temporal frequency at which the injection is performed can bealtered by increasing a frequency at which the injection is performed;and the different amount of buffer gas can be pumped into the gasmixture by pumping less buffer gas into the gas mixture than was pumpedbefore it was determined that any of the one or more monitored operatingcharacteristics will be out of an acceptable range.

The selected gas maintenance scheme can be applied to the gas dischargesystem by applying the selected gas maintenance scheme after each of thegas discharge chambers is filled with its respective gas mixture. Themethod can also include, after the selected gas maintenance scheme isapplied to the gas discharge system, performing a refill scheme to thegas discharge chamber. The refill scheme can include emptying each ofthe gas discharge chambers of the gas discharge system; and refillingeach gas discharge chamber with fresh gas mixture.

The gas maintenance scheme can include one or more of the followingparameters: an amount of a component gas that is to be pumped into thegas mixture; and a temporal frequency at which an injection of thecomponent gas into the gas mixture is performed. The component gas canbe a buffer gas (such as an inert gas) of the gas mixture, and the gasmixture can include a gain medium.

One of the gas maintenance schemes can be a refill scheme that includesemptying at least one of the gas discharge chambers and refilling thatemptied gas discharge chamber with fresh gas mixture.

In other general aspects, a gas discharge light source that includes oneor more gas discharge chambers is operated. A gas discharge chamber isfilled with a gas mixture, the gas discharge chamber housing an energysource. A pulsed energy is supplied to the gas mixture by activating theenergy source to thereby produce a pulsed amplified light beam. One ormore properties of the gas discharge chamber are determined, and aninjection scheme is selected from among a plurality of possibleinjection schemes based on the determined one or more properties of thegas discharge chamber. The selected injection scheme is applied to thegas discharge chamber. An injection scheme includes one or moreparameters related to adding one or more supplemental gas mixtures tothe gas discharge chamber.

In other general aspects, a gas discharge light source includes: a gasdischarge system that includes one or more gas discharge chambers, eachgas discharge chamber housing an energy source and containing a gasmixture that includes a gain medium; and a gas maintenance system. Thegas maintenance system includes a gas supply system; a monitoringsystem; and a control system coupled to the gas supply system and to themonitoring system. The control system is configured to: provide a signalto activate each energy source to thereby produce a pulsed amplifiedlight beam from its gas discharge chamber; receive information from themonitoring system and determine one or more properties of the gasdischarge system based on this received information; select a gasmaintenance scheme from among a plurality of possible schemes based onthe determined one or more properties of the gas discharge system; andprovide a signal to the gas supply system to thereby apply the selectedgas maintenance scheme to the gas discharge system. A gas maintenancescheme includes one or more parameters related to adding one or moresupplemental gas mixtures to the gas discharge chambers of the gasdischarge system.

In other general aspects, a gas discharge light source is operated usinga method. The gas discharge light source includes a gas discharge systemthat includes one or more gas discharge chambers, each gas dischargechamber housing an energy source. The method includes: filling each ofthe gas discharge chambers in the gas discharge system with a respectivegas mixture; for each gas discharge chamber, supplying a pulsed energyto the respective gas mixture by activating its energy source to therebyproduce a pulsed amplified light beam from the gas discharge chamber;monitoring one or more operating characteristics of the gas dischargelight source; determining whether any of the one or more monitoredoperating characteristics will be out of an acceptable range at a futuretime; and if it is determined that any of the one or more monitoredoperating characteristics would be out of an acceptable range at thefuture time, then selecting a restore gas maintenance scheme andapplying the selected restore gas maintenance scheme to the gasdischarge system by increasing a relative amount a component gas in thegas mixture of at least one of the gas discharge chambers.

Implementations can include one or more of the following features. Forexample, the component gas can include a buffer gas such as, forexample, an inert gas such as neon.

A relative amount of the component gas in the gas mixture of at leastone of the gas discharge chambers can be increased by applying a restoregas injection scheme to the at least one gas discharge chamber. Therestore injection scheme can be applied by increasing a temporalfrequency at which an injection of the component gas is performed. Therestore injection scheme can be applied by pumping more component gasinto the gas mixture of the at least one gas discharge chamber than waspumped before it was determined that any of the one or more monitoredoperating characteristics will be out of an acceptable range. Or, therestore injection scheme can be applied by both increasing a temporalfrequency at which an injection of the component gas is performed andpumping more component gas into the gas mixture of the at least one gasdischarge chamber than was pumped before it was determined that any ofthe one or more monitored operating characteristics will be out of anacceptable range.

The one or more operating characteristics of the gas discharge lightsource can be monitored by monitoring one or more of: a pulsed energythat is supplied to the gas mixture of at least one of the gas dischargechambers; and an energy of the pulsed amplified light beam output fromat least one of the gas discharge chambers. The one or more operatingcharacteristics can be monitored by measuring one or more of thefollowing characteristics of the gas discharge light source: a change inthe pulsed energy supplied to the gas mixture of at least one of the gasdischarge chambers over time; and a change in the energy of the pulsedamplified light beam output from at least one of the gas dischargechambers over time.

One or more operating characteristics of the gas discharge light sourcecan be monitored by calculating values of the operating characteristics,and determining whether any of the one or more monitored operatingcharacteristics will be out of the acceptable range at a future timecomprises determining whether any of the calculated values of theoperating characteristics will be out of the acceptable range at afuture time. The values of the operating characteristics can becalculated by calculating average values of the operatingcharacteristics.

A relative amount of the component gas in the gas mixture of at leastone of the gas discharge chambers can be increased by applying a refillscheme to the at least one gas discharge chamber. The refill schemeincludes: purging the gas mixture from the at least one gas dischargechamber and filling the at least one gas discharge chamber with a freshgas mixture that includes the component gas.

Determining whether any of the one or more monitored operatingcharacteristics will be out of the acceptable range at a future time caninclude determining whether any of the one or more monitored operatingcharacteristics is likely to be out of the acceptable range at a futuretime.

Determining whether any of the one or more monitored operatingcharacteristics will be out of the acceptable range at a future time caninclude: determining a rate of change of each of the one or moremonitored operating characteristics; and determining whether the rate ofchange for each of the one or more monitored operating characteristicsindicates whether that monitored operating characteristic is likely tobe out of the acceptable range at the future time.

The method can include determining whether any of the one or moremonitored operating characteristics will be out of another acceptablerange at a future time, and if it is determined that any of the one ormore monitored operating characteristics will be out of the otheracceptable range at a future time, then applying a refill scheme to atleast one gas discharge chamber. The refill scheme includes: purging thegas mixture from the at least one of the gas discharge chambers, andfilling the purged gas discharge chamber with fresh gas mixture thatincludes the component gas.

The one or more operating characteristics of the gas discharge lightsource can be monitored while the pulsed amplified light beam isproduced.

The gas discharge system can include a first gas discharge chamberhousing a first energy source and a second gas discharge chamber housinga second energy source. Each of the gas discharge chambers can be filledwith a respective gas mixture by filling the first gas discharge chamberwith a first gas mixture and filling the second gas discharge chamberwith a second gas mixture. The selected restore gas maintenance schemecan be applied to the gas discharge system by increasing a relativeamount of a component gas in a first gas mixture of the first gasdischarge chamber and increasing a relative amount of a component gas ina second gas mixture of the second gas discharge chamber.

The gas discharge chamber can be filled with the respective gas mixtureby filling the gas discharge chamber with a mixture of a gain medium anda buffer gas. The gas discharge chamber can be filled with the mixtureof the gain medium and the buffer gas by filling the gas dischargechamber with a gain medium that includes a noble gas and a halogen, anda buffer gas that includes an inert gas. The inert gas can includehelium or neon and the component gas can include the inert gas.

In other general aspects, a gas discharge light source includes a gasdischarge system that includes one or more gas discharge chambers, eachgas discharge chamber housing an energy source and containing a gasmixture that includes a gain medium; and a gas maintenance system. Thegas maintenance system includes a gas supply system; a monitoringsystem; and a control system coupled to the gas supply system and to themonitoring system. The control system is configured to: provide a signalto activate each energy source to thereby produce a pulsed amplifiedlight beam from its gas discharge chamber; receive information from themonitoring system and determine one or more operating characteristics ofthe gas discharge system based on this received information; determinewhether any of the operating characteristics will be out of anacceptable range at a future time; and if it is determined that any ofthe operating characteristics will be out of an acceptable range at afuture time, then selecting a restore gas maintenance scheme andproviding a signal to the gas supply system to thereby apply theselected restore gas maintenance scheme to the gas discharge system. Therestore gas maintenance scheme increases a relative amount of acomponent gas in the gas mixture of at least one of the gas dischargechambers.

The method and system described herein effectively and continuouslydetermine whether the gas component could be conserved at any given timebased on the age of the gas discharge chamber. The implementation of themethod and system can lead to a direct reduction of the gas component(such as neon) by at least 50% and potentially as much as 75% whencompared with a method and system that lacks a gas conservation scheme.Conservation of the gas component is done with the comfort of knowingthat a restore gas maintenance system is in use that monitors whetherthe gas conservation scheme will lead to an unacceptable reduction inquality of the output of the gas discharge chamber at a future time. Themethod and system described herein therefore reduces the volumerequirements for the gas component and can save the customer whopurchases the system (and method) hundreds of thousands of dollars foreach gas discharge light source 100.

DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram of a gas discharge light source that producesa pulsed light beam directed to an output apparatus;

FIG. 2 is a block diagram of an exemplary output apparatus;

FIG. 3 is a block diagram of an exemplary gas discharge light source;

FIG. 4 is a block diagram of an exemplary control system for performinggas maintenance and restore gas maintenance schemes;

FIG. 5 is a flow chart of a procedure performed by the gas dischargelight source of FIGS. 1 and 2 for a gas maintenance scheme, which canalso include a sub-procedure for a restore gas maintenance scheme;

FIG. 6 is a flow chart of exemplary procedures performed by the controlsystem of the light source of FIGS. 1 and 2 for determining whether amonitored operating characteristic will be out of an acceptable range ata future time and selecting a restore gas maintenance scheme;

FIG. 7 is a flow chart of exemplary procedures are sequentiallyperformed by the control system of the light source of FIGS. 1 and 2 fordetermining how the operating characteristic changes with the usage ofthe light source; and

FIG. 8 is a chart of graphs of the various outputs during one of theprocedures of FIG. 7.

DESCRIPTION

Referring to FIG. 1, a gas discharge light source 100 includes a gasdischarge system 105 and a gas maintenance system 110. The light source100 is configured as a part of an optical system 115 that supplies apulsed light beam 120 that is directed to an output apparatus 125 (suchas a photolithography exposure apparatus that patterns microelectronicfeatures on a wafer, as shown in FIG. 2). The pulsed light beam 120 canbe directed through a beam preparation system 130 placed between the gasdischarge light source 100 and the output apparatus 125.

The gas discharge system 105 includes one or more gas discharge chambers135. Each gas discharge chamber 135 houses an energy source 140 andcontains a gas mixture 145 that includes a gain medium, among othergases. For example, the gas mixture includes a mixture of a gain mediumand a buffer gas. The gain medium is the laser-active entity within thegas mixture, and it can be either a single atom or a molecule or apseudo-molecule. Thus, a population inversion occurs in the gain mediumvia stimulated emission by pumping the gas mixture (and therefore thegain medium) with an electric discharge from the energy source 140. Thegain medium typically includes a noble gas and a halogen, while thebuffer gas typically includes an inert gas. The noble gas includes, forexample, argon, krypton, or xenon; the halogen includes, for example,fluorine; and the inert gas includes, for example, helium or neon.

The gas maintenance system 110 includes a gas supply system 150; amonitoring system 155; and a control system 160. The gas supply system150 includes one or more gas control valves as well as one or more gassources.

The control system 160 is coupled to the gas supply system 150 and tothe monitoring system 155. The control system 160 is coupled to a device(such as the gas supply system 150 or the monitoring system 155)external to the control system 160 if the control system 160 isconnected (either wired or wirelessly) so that information can be freelypassed between the control system 160 and that particular device. Thecontrol system 160 can additionally be coupled to one or more of: otherdevices of the gas discharge light source 100, devices of the beampreparation system 130, and/or devices within the output apparatus 125.For example, the control system 160 can include sub-systems that monitorand control other aspects of the gas discharge light source 100, suchas, for example, monitoring a spectral feature of the pulsed light beam120 or controlling a spectral feature of the pulsed light beam 120.

Additionally, although the control system 160 is represented as a box inwhich all of the components appear to be co-located, it is possible forthe control system 160 to be made up of components that are physicallyremote from each other.

The control system 160 is configured to provide a signal to activateeach energy source 140 of each gas discharge chamber 135. The gasdischarge chamber 135 can therefore produce a pulsed amplified lightbeam (such as the pulsed light beam 120 or an intermediate light beam)if the gas discharge chamber 135 includes additional optical feedback.

The control system 160 receives information from the monitoring system155 and determines one or more properties of the gas discharge system105 based on this received information. The control system 160 selects agas maintenance scheme from among a plurality of possible schemes basedon the determined one or more properties of the gas discharge system105. The control system 160 provides a signal to the gas supply system150 to cause the gas supply system 150 to apply the selected gasmaintenance scheme to the gas discharge system 105. For example, the gassupply system 150 could actuate or control aspects of one or more valvesfor adjusting a relative amount of gas that is provided from the gassources.

A gas maintenance scheme includes one or more parameters related toadding one or more supplemental gas mixtures to the gas dischargechambers 135 of the gas discharge system 105.

The gas maintenance system 110 is a gas management system for the gasdischarge system 105, and specifically for the gas discharge chambers135. The gas maintenance system 110 is configured to reduce theconsumption or use of at least one component gas (such as a buffer gas,which can be an inert gas such as neon) of the gas mixture used in thegas discharge chambers 135 in order to conserve the component gas whilemaintaining adequate performance of the gas discharge system 105. Thegas maintenance system 110 uses information about the gas dischargesystem (such as, for example, an age of a gas discharge chamber 135) todetermine an appropriate gas maintenance scheme. For example, thecontrol system 160 may determine that the gas discharge system 105 is ayoung system, which tend to require and consume a high amount of gasmixture. In this case, the control system 160 can select a standard gasmaintenance scheme that does not conserve any of the component gases. Asanother example, the control system 160 may determine that the gasdischarge system is a middle-aged system, which tend to require andconsume a relatively lower amount of gas mixture. In this case, thecontrol system 160 can select a conservation gas maintenance scheme thatconserves as much of the component gas as possible while stillmaintaining suitable output parameters for the pulsed light beam 120. Asyet another example, the control system 160 may determine that the gasdischarge system is an older system, which tend to require and consumean amount of gas mixture that is less than the amount required by theyoung system yet greater than the amount required by the middle-agedsystem. In this case, the control system 160 can select a mid-levelconservation gas maintenance scheme that conserves some of the componentgas while still maintaining suitable output parameters for the pulsedlight beam 120.

By tailoring the gas maintenance scheme to the age of the gas dischargechamber 135, the customer can obtain conservation of component gas whilereducing an amount of downtime of the gas discharge light source 100. Atthe same time, the performance of the gas discharge light source 100 canbe maintained at acceptable levels, even though the gas maintenancescheme may be designed to tolerate a greater depletion of gain medium,which can otherwise reduce an amount of energy and power in the pulsedlight beam 120 provided to the output apparatus 125. Downtime of the gasdischarge light source 100 is the time during which no pulsed light beam120 is being provided to the output apparatus 125.

In other implementations, the control system 160 receives one or moreoperating characteristics of the gas discharge light source 105 from themonitoring system 155. The control system 160 determines whether any ofthe one or more monitored operating characteristics will be out of anacceptable range at a future time. If the control system 160 determinesthat any of the one or more monitored operating characteristics will beout of an acceptable range at a future time, then the control system 160selects a restore gas maintenance scheme. The control system 160 thensends a signal to the gas supply system 150 to apply the selectedrestore gas maintenance scheme to the gas discharge system 105 byincreasing a relative amount a component gas in the gas mixture 145 ofat least one of the gas discharge chambers 135.

The restore gas maintenance scheme can be used as a fault tolerancesystem for when the gas maintenance system 110 is operating in aconservation gas maintenance scheme. For example, the control system 160can receive from the monitoring system 155 a prediction of a pulsedenergy that will be required to be supplied to the gas mixture 145 of agas discharge chamber 135 at a future time. As another example, thecontrol system 160 can receive from the monitoring system 155 aprediction of an energy of a pulsed light beam output from the gasdischarge chamber 135 at a future time. If either of these predictionsis outside of a range of acceptable values, then it could be becausethere is not enough component gas in the gas mixture 145 (due to theimplementation of the conservation gas maintenance scheme). Thus, ifeither or both of these predictions are outside of their range ofacceptable values, then the control system 160 can halt the conservationgas maintenance scheme by switching to the restore gas maintenancescheme, which is a scheme that increases an amount of the component gaswithin the gas mixture 145. Moreover, the determination regardingwhether either or both of these predictions are outside their range ofacceptable values is an automatic determination (in that it does notrequire a manual or human intervention). By providing this additionalfault tolerance system, it is possible to be more aggressive inselecting when to run a conservation gas maintenance scheme.

Details about an exemplary gas discharge light source 300 are providedwith reference to FIG. 3. Details about the conservation gas maintenancescheme are provided with reference to FIG. 5, and a description of thefault tolerance system procedure is also found in FIG. 5. Referring toFIG. 3, an exemplary gas discharge light source 300 includes a gasdischarge system 305 that is a dual-chamber pulsed output design and agas supply system 350.

The gas discharge system 305 includes a master oscillator (MO) having aMO gas discharge chamber 335A and a power amplifier (PA) having a PA gasdischarge chamber 335B. The MO gas discharge chamber 335A includes twoelongated electrodes 340A that provide a source of pulsed energy to thegas mixture within the MO gas discharge chamber 335A. The PA gasdischarge chamber 335B includes two elongated electrodes 340B thatprovide a source of pulsed energy to the gas mixture within the PA gasdischarge chamber 335B.

The master oscillator (MO) provides a pulsed amplified light beam(called a seed light beam) 370 to the power amplifier (PA). The MO gasdischarge chamber 335A houses the gas mixture that includes a gainmedium in which amplification occurs and the MO includes an opticalfeedback mechanism such as an optical resonator. The PA gas dischargechamber 335B houses the gas mixture that includes a gain medium in whichamplification occurs when seeded with the seed laser beam 370 from theMO. If the PA is designed as a regenerative ring resonator then it isdescribed as a power ring amplifier (PRA), and in this case, enoughoptical feedback can be provided from the ring design. The MO enablesfine tuning of spectral parameters such as the center wavelength and thebandwidth at relatively low output pulse energies (when compared withthe output of the PA). The PA receives the output (the seed light beam370) from the MO and amplifies this output to attain the necessarypowers for output to use in the output apparatus 125 (for example, forphotolithography).

The MO gas discharge chamber 335A also includes a fan for circulatingthe gas between the electrodes 340A. A laser resonator is formed betweena spectral feature selection system 375 on one side of the MO gasdischarge chamber 335A and an output coupler 380 on a second side of theMO gas discharge chamber 335A.

The gas mixture (for example, 145, 345A, 345B) used in the dischargechamber can be a combination of suitable gases for producing anamplified light beam around the required wavelengths and bandwidth. Forexample, the gas mixture can include argon fluoride (ArF), which emitslight at a wavelength of about 193 nm, or krypton fluoride (KrF), whichemits light at a wavelength of about 248 nm.

The PA can also include a beam return (such as a reflector) 385 thatreturns (via reflection, for example) the light beam back into the PAgas discharge chamber 335B to form a circulating and looped path (inwhich the input into the ring amplifier intersects the output out of thering amplifier). The PA gas discharge chamber 335B includes a fan forcirculating the gas mixture 345B between the electrodes 340B. The seedlight beam 370 is amplified by repeatedly passing through the PA.Spectral features of the seed light beam 370 are determined by theconfiguration of the MO, and these spectral features can be adjusted byadjusting a light beam that is produced within the MO.

The gas supply system 350 includes one or more gas sources 351A, 351B,351C (such as sealed gas bottles or canisters) and a valve system 352.As discussed above, each of the gas discharge chambers 335A, 335Bcontains a mixture of gases (a gas mixture 145). As an example, the gasmixture 145 can contain a halogen, for example, fluorine, along withother gases such as argon, neon, and possibly others in differentpartial pressures that add up to a total pressure P. Thus, the one ormore gas sources 351A, 351B, 351C, etc. are connected to the MO gasdischarge chamber 335A and the PA gas discharge chamber 335B through aset of valves within the valve system 352. In this way, gas can beinjected into the gas discharge chamber 335A, 335B with specificrelative amounts of components of the gas mixture. For example, if thegain medium used in the gas discharge chambers 335A, 335B is argonfluoride (ArF), then one of the gas sources 351A can contain a mixtureof gases including the halogen fluorine, the noble gas argon, and one ormore other rare gases such as buffer gases (inert gas such as neon).This sort of mixture can be referred to as a tri-mix. In this example,another of the gas sources 351B can contain a mixture of gases includingargon and one or more other gases but none of the fluorine. This sort ofmixture can be referred to as a bi-mix.

The control system 160 can send one or more signals to the valve system352 to cause the valve system 352 to transfer gases from specific gassources 351A, 351B, 351C into the gas discharge chambers 335A, 335B in arefill or an inject scheme. Alternatively, or additionally, the controlsystem 160 can send one or more signals to the valve system 352 to causethe valve system 352 to bleed gas from the gas discharge chambers 335A,335B when necessary, and such bled gas can be vented to a gas dumprepresented as 390.

During operation of a gas discharge light source 300, the fluorine ofthe argon fluoride molecule (which provides the gain medium for lightamplification) within the gas discharge chambers 335A, 335B is consumedand over time this reduces the amount of light amplification (andtherefore the energy of the amplified light beam 370, 320) produced bythe respective gas discharge chamber 335A, 335B. Moreover, duringoperation of the gas discharge light source 300, contaminants can enterthe gas discharge chambers 335A, 335B. Accordingly, it is necessary toeither inject gases from one or more of the gas sources 351A, 351B,351C, etc. into the gas discharge chambers 335A, 335B in order to flushcontaminants out of the gas discharge chambers 335A, 335B. For example,it is possible to flush the gas discharge chambers using the bi-mix fromthe gas source 351B. However, each flush uses non-negligible amounts ofgas components (such as the buffer gas, which can be an inert gas suchas neon) within the bi-mix. Accordingly, the gas maintenance procedureand system described herein strives to reduce one or more of thefrequency and size of the bi-mix injections. Because of this reductionin the frequency and/or size of the bi-mix injections, the gas dischargechambers 335A, 335B would need to sustain a lack of injections of thehalogen (such as fluorine) for longer periods of time and thus wouldoperate with higher amounts of contaminants. However, the gas dischargelight source may suffer greatly in efficiency and power output dependingon the age of the gas discharge chamber 335A, 335B if the gasmaintenance procedure and system did not take into account the age ofthe gas discharge chamber 335A, 335B when selecting or choosing afrequency, timing, or size of a gas injection (a gas maintenancescheme). Thus, the gas maintenance procedure and system described hereinselective chooses one or more of the frequency and size of a gasinjection based on one or more properties (such as age) of the gasdischarge system 105, 305. Thus, the gas maintenance procedure andsystem described herein enables different levels of a reduction in useof a gas component that is used in the gas mixture 145 as a function ofthe age of the gas discharge chamber 135, 335A, 335B.

Additionally, the gas maintenance procedure and system described hereinalso includes a safety net sub-process (the restore maintenance schemedescribed herein) that monitors the operating characteristics of the gasdischarge light source 105, 305 to determine whether a lack of gasinjection would cause an unacceptable event (such as an unacceptablerise in energy required to be input to the gas mixture 145 to obtain aspecific output energy in the pulsed light beam 320, 370 or anunacceptable drop in the output energy of the pulsed light beam 320,370) at a future time. These gas maintenance and restore schemes can beperformed automatically, that is, without the assistance from a personsuch as a field service engineer. The restore maintenance scheme can beused to restore the gas injection frequency or size to original levelsor to request a refill (which is described herein).

A plurality of gas sources 351A, 351B, 351C, etc. are needed because thefluorine in the gas source 351A is at a particular partial pressure thatis typically higher than that desired for laser operation. In order toadd the fluorine to the MO gas discharge chamber 335A or the PA gasdischarge chamber 335B at a desired lower partial pressure, the gas inthe gas source 351A can be diluted, and the non-halogen containing gasin the gas source 351B can be used for this purpose.

Although not shown, the valves of the valve system 352 can include aplurality of valves assigned to each of the gas discharge chambers 335A,335B; for example, an injection valve that allows gas to pass into andout of each gas discharge chamber 335A, 335B at a first rate, and achamber fill valve that allows gas to pass into and out of each gasdischarge chamber 335A, 335B at a second rate that is distinct from (forexample, faster) the first rate.

When a refill scheme is performed on the gas discharge chambers 335A,335B, all of the gas in each of the gas discharge chambers 335A, 335B isreplaced by, for example, emptying the gas discharge chamber 335A, 335B(by bleeding the gas mixture out to the gas dump 390) and then refillingthat gas discharge chamber 335A, 335B with a fresh gas mixture. Therefill is performed with the goal of obtaining a specific pressure andconcentration of fluorine in each gas discharge chamber 335A, 335B.

When an injection scheme is performed on the gas discharge chambers335A, 335B, the gas discharge chambers 335A, 335B are not emptied or areonly bled a small amount before a gas mixture is injected into the gasdischarge chambers 335A, 335B.

Refill schemes and injection schemes are considered to be gasmaintenance schemes applied to the gas discharge system (105 or 305).

Referring to FIG. 5, a procedure 500 is performed by the gas dischargelight source 100, 300 for managing an amount of a gas component (such asa buffer gas which can be an inert gas such as neon) within the gasmixture 145. The procedure 500 can be performed while energy is suppliedto the gas mixture 145 via the energy source 140 and the amplified lightbeam 120 is being provided to the output apparatus 125. Alternatively,the procedure 500 can be performed while energy is not being supplied tothe gas mixture 145 via the energy source 140 and therefore theamplified light beam 120 is not produced.

The procedure 500 includes filling each of the gas discharge chambers135, 335A, 335B in the gas discharge system 105, 305 with a respectivegas mixture 145, 345A, 345B (505). For each gas discharge chamber 135,335A, 335B, a pulsed energy is supplied to the respective gas mixture145 by activating its energy source 140 to thereby produce a pulsedamplified light beam 120, 370, 320 from the gas discharge chamber 135,335A, 335B (510).

The control system receives information from the monitoring system 115and determines one or more properties of the gas discharge system 105,305 (515). The one or more properties of the gas discharge system 105,305 can include, for example, the age of the gas discharge chamber 135,335, 335B. The age of the gas discharge chamber 135, 335A, 335B can bedetermined by probing a shot count associated with that particularchamber 135, 335A, 335B. This shot count is reset manually to a new (forexample, zero) value, in the control system 160, when a new chamber isinstalled. The shot count is a number that corresponds to an integermultiple of the number of pulses of energy supplied to the gas mixture145 in the gas discharge chamber 135, 335A, 335B.

The control system 160 selects a gas maintenance scheme from among aplurality of possible gas maintenance schemes based on the determinedone or more properties of the gas discharge system (520). The selectedgas maintenance scheme is then applied to the gas discharge system 105,305 (525). For example, the control system 160 sends a signal to the gassupply system 150, 350 to open or close one or more of the valves in thevalve system 352 to thereby control relative amounts of components ofthe gas mixture 145.

Each gas maintenance scheme includes one or more parameters related toadding one or more supplemental gas mixtures to the gas dischargechambers 135, 335A, 335B of the gas discharge system 105, 305.

As discussed above, the gas discharge chamber 135 is filled with the gasmixture (505) by filling the gas discharge chamber 135 with a mixture ofa gain medium and a buffer gas that make up the gas mixture. The gainmedium can include a noble gas such as argon, krypton, or xenon, and ahalogen such as fluorine, and the buffer gas can include an inert gassuch as helium or neon.

The pulsed energy can be supplied to the respective gas mixture 145(510) by activating its energy source 140, 340A, 340B by applying apulsed voltage to the pair of electrodes within the gas dischargechamber 135, 335A, 335B so that an electrical stimulation is applied tothe halogen (for example, fluorine) within the gas mixture 145.

One of the properties of the gas discharge system that is determined isan age of at least one of the gas discharge chambers 135, 335A, 335B ofthe gas discharge system 105, 305. The age can be determined based onone or more of: how many times that gas discharge chamber has beenfilled with the gas mixture and how often the energy source of that gasdischarge chamber has been activated.

The control system 160 may determine that an age of at least one of thegas discharge chambers 135, 335A, 335B is in a first range. In thiscase, the control system 160 selects a standard gas injection scheme,and sends a signal to the gas supply system 150, 350 to apply theselected gas injection scheme to the gas discharge system 105, 305 bypumping at least a first amount of a buffer gas (for example, neon) intothe gas mixture 145 of the gas discharge chamber 135, 335A, 335B. Thefirst range could represent an age of a gas discharge chamber 135, 335A,335B that is early in life, which means that the gas discharge chamber135, 335A, 335B has been in operation for a very little amount of timewhen compared to its total life span. The first amount of buffer gas canbe the standard amount (which is based on frequency and size of gasinjection from the gas supply system 150, 350) that would be appliedwithout attempting to conserve any of the gas components of the gasmixture.

The control system 160 may determine that an age of at least one of thegas discharge chambers 135, 335A, 335B is in a second range. In thiscase, the control system 160 selects a conservation gas injectionscheme, and sends a signal to the gas supply system 150, 350 to applythe conservation gas injection scheme to the gas discharge system 135,335A, 335B by pumping a second amount of the buffer gas (for example,neon) into the gas mixture 145 of the gas discharge chamber 135, 335A,335B. The second amount of buffer gas is lower than the first andstandard amount, which means that either or both of a frequency and sizeof gas injection is reduced in the conservation gas injection scheme.The second range represents an age of the gas discharge chamber 135,335A, 335B that is mid-life; which means that the gas discharge chamber135, 335A, 335B has been in operation for an amount of time that is nearto a mid-way point of its total life span.

The control system 160 may determine that an age of at least one of thegas discharge chambers 135, 335A, 335B is in a third range. In thiscase, the control system 160 selects another conservation gas injectionscheme (which may be distinct from the conservation gas injection schemediscussed above), and the control system 160 sends a signal to the gassupply system 150, 350 to apply the other conservation gas injectionscheme to the gas discharge system 135, 335A, 335B by pumping a thirdamount of the buffer gas (for example, neon) into the gas mixture 145 ofthe gas discharge chamber 135, 335A, 335B. The third amount of buffergas is lower than the first (and standard) amount but is greater thanthe second amount. This means that either or both of a frequency andsize of gas injection is reduced in the other conservation gas injectionscheme when compared with the standard gas injection scheme but is stillgreater than the frequency and/or size of gas injection used in theconservation gas injection scheme discussed above. The third rangerepresents an age of the gas discharge chamber 135, 335A, 335B that islate in life, which means that the gas discharge chamber 135, 335A, 335Bhas been in operation for an amount of time that is past the mid-waypoint of its total life span.

The first range of ages can be a value less than or equal to a lowervalue, the second range of ages can be a value that is greater than thelower value, and the third range of ages can be a value that is greaterthan an upper value. For example, the first range can be 0-1 billionpulses of the energy source 140, the second range can be 1-25 billionpulses of the energy source 140, and the third range can be greater than25 (for example, 25-30) billion pulses of the energy source 140. In thisexample, a young gas discharge system 105 would therefore be a system105 that has only operated for 1 billion or fewer pulses, a mid-age gasdischarge system 105 would be a system 105 that has operated for 1-25billion pulses, and an old gas discharge system 105 would be a system105 that has operated for greater than 25 billion pulses.

The first range can be a first value of the age and the second range canbe a second value of the age. The third range can be a third value ofthe age.

The second range can be distinct from the first range.

In other implementations, alternatively or additionally, the standardgas injection scheme includes pumping or performing an injection of thebuffer gas into the gas mixture of the gas discharge chamber at a firsttemporal frequency; the conservation gas injection scheme includespumping or performing an injection of the buffer gas into the gasmixture of the gas discharge chamber at a second temporal frequency thatis different from the first temporal frequency. The second temporalfrequency is less than the first temporal frequency.

In some implementations, the temporal frequency is measured in pulses orshots. For example, the standard gas injection scheme could includeperforming an injection of the buffer gas into the gas mixture of thegas discharge chamber each time a certain number of pulses are suppliedto the energy source 140. For example, for every one million pulses ofthe pulsed light beam 120 that have been produced (or after one millionpulses of energy are supplied to the energy source 140), an injection isperformed under the standard gas injection scheme. As another example,in the conservation gas injection scheme, an injection of the buffer gasinto the gas mixture occurs with every two million pulses of the pulsedlight beam 120 that have been produced. In this example, the injectionof the buffer gas could happen between pulses.

In other implementations, the temporal frequency is measured in timewithout taking into account the number of pulses or shots. For example,the standard gas injection scheme could include performing an injectionof the buffer gas into the gas mixture every X minutes while theconservation gas injection scheme could include performing an injectionof the buffer gas into the gas mixture every Y minutes, where Y is lessthan X. In this example, the injection of the buffer gas could happenafter the energy is stop being supplied to the energy source 140.

As discussed above, the buffer gas is injected into the gas mixture ofthe gas discharge chamber; however, because the buffer gas is mixed withother gas components as discussed above, this also includes injectingone or more components of the gain medium into the gas mixture of the atleast one gas discharge chamber.

The control system 160 can also receive information from the monitoringsystem 155 that provides one or more operating characteristics of thegas discharge light source 105, 305. The control system 160 determineswhether any of the one or more monitored operating characteristics isout of an acceptable range. And, if the control system 160 determinesthat any of the one or more monitored operating characteristics is outof an acceptable range, then the control system 160 selects the restoregas maintenance scheme and sends a signal to the gas supply system 150,350 to apply the selected restore gas maintenance scheme to the gasdischarge system 105, 305.

The gas supply system 150, 350 applies the selected restore gasmaintenance scheme to the gas discharge system 105, 305 by applying oneor more of a restore injection scheme and a refill scheme to the gasdischarge system 105, 305.

In the restore injection scheme, the gas supply system 150, 350 performsan injection of a buffer gas into the gas mixture 145 of at least one ofthe gas discharge chambers 135, 335A, 335B that increases a relativeamount of the buffer gas in the gas mixture 145 of the gas dischargechamber 135, 335A, 335B. For example, in the restore injection scheme,the gas supply system 150, 350 can alter a temporal frequency at whichthe gas injection is performed or can pump a different amount of buffergas into the gas mixture 145 than was pumped before it was determinedthat any of the one or more monitored operating characteristics is outof an acceptable range.

The gas supply system 150, 350 can alter the temporal frequency at whichthe gas injection is performed by increasing a frequency at which thegas injection is performed.

The gas supply system 150, 350 can pump the different amount of buffergas into the gas mixture 145 than was pumped before it was determinedthat any of the one or more monitored operating characteristics is outof an acceptable range by pumping less buffer gas into the gas mixture145 than was pumped before it was determined that any of the one or moremonitored operating characteristics is out of an acceptable range.

The gas supply system 150, 350 applies the selected gas maintenancescheme to the gas discharge system 105, 305 after each of the gasdischarge chambers 135, 335A, 335B is filled with its respective gasmixture 145. Moreover, the control system 160 may instruct the gassupply system 150, 350 to perform a refill scheme on the gas dischargechamber 135, 335A, 335B after the gas supply system 150, 350 has appliedthe selected gas maintenance scheme to the gas discharge system 105,305. The refill scheme includes emptying each of the gas dischargechambers 135, 335A, 335B of the gas discharge system 105, 305; andrefilling each gas discharge chamber 135, 335A, 335B with a fresh gasmixture from one or more of the gas sources 351A, 351B, 351C, etc. Forexample, the gas sources 351A and 351B can be used for refilling, andthe valve system 352 can be used to adjust the relative flow ratesbetween the gas mixtures that flow from the respective gas sources 351A,351B.

Thus, as discussed above, a gas maintenance scheme that is applied tothe gas discharge system 105, 305 includes one or more of the followingparameters: an amount of a component gas (such as a buffer gas) that isto be pumped into the gas mixture 145; and a temporal frequency at whichan injection of the component gas (such as the buffer gas) into the gasmixture 145 is performed.

Additionally, at least one of the gas maintenance schemes could be arefill scheme that includes emptying at least one of the gas dischargechambers 135, 335A, 335B and refilling that emptied gas dischargechamber 135, 335A, 335B with fresh gas mixture 145.

As discussed above, in addition to the gas maintenance scheme describedabove, it is possible to perform a restore maintenance scheme in orderto ensure that the gas discharge light source 100, 300 continues tooperate at levels that are acceptable to the output apparatus 125.

Referring again to FIG. 5, a restore maintenance sub-procedure 530 isperformed, as follows, during operation of or following the gasmaintenance scheme 500 and is performed by the gas discharge lightsource 100, 300. The control system 160 receives one or more operatingcharacteristics of the gas discharge light source 100, 300 that aremeasured or monitored by the monitoring system 155 (535). The controlsystem 160 analyzes those operating characteristics of the gas dischargelight source 100, 300 (540) and, based on this analysis, determineswhether any of the one or more monitored operating characteristics willbe out of an acceptable range (545) at a future time. If the controlsystem 160 determines that any of the one or more monitored operatingcharacteristics will be out of an acceptable range (545) at a futuretime, then the control system 160 selects a particular restore gasmaintenance scheme (550) from a set of possible restore gas maintenanceschemes, and sends a signal to the gas supply system 150, 350 to applythe selected restore gas maintenance scheme to the gas discharge system105, 305 (555) by increasing a relative amount a component gas (such asthe buffer gas of neon) in the gas mixture 145 of at least one of thegas discharge chambers 135, 335A, 335B. The relative amount of thecomponent gas in the gas mixture 145 can be increased by applying arestore gas injection scheme to the at least one gas discharge chamber135, 335A, 335B.

The restore injection scheme can be applied to the gas discharge chamber135, 335A, 335B by increasing a temporal frequency at which an injectionof the component gas is performed, as discussed above. Alternatively oradditionally, the restore injection scheme can be applied to the gasdischarge chamber 135, 335A, 335B by pumping more component gas into thegas mixture 145 of the gas discharge chamber 135, 335A, 335B than waspumped before it was determined that any of the one or more monitoredoperating characteristics will be out of an acceptable range at a futuretime.

An exemplary operating characteristic of the gas discharge light source100, 300 that can be monitored is a pulsed energy that is required to besupplied to the gas mixture 145 in at least one of the gas dischargechambers 135, 335A, 335B. Another exemplary operating characteristic ofthe gas discharge light source 100, 300 that can be monitored is anenergy of the pulsed amplified light beam 120, 320, 370 output from atleast one of the gas discharge chambers 135, 335A, 335B.

These operating characteristics can be monitored by measuring one ormore characteristics of the gas discharge light source 100, 300.Measured characteristics include the pulsed energy supplied to the gasmixture of at least one of the gas discharge chambers; an energy of thepulsed amplified light beam output from at least one of the gasdischarge chambers; a change in the pulsed energy supplied to the gasmixture of at least one of the gas discharge chambers over time; and achange in the energy of the pulsed amplified light beam output from atleast one of the gas discharge chambers over time. The pulsed energysupplied to the gas mixture 145 is directly correlated to a voltage thatis applied to the energy source 140, 340A, 340B in the gas dischargechamber 135, 335A, 335B.

By monitoring the change in an operating characteristic (such as theenergy of the pulsed amplified light beam output from at least one ofthe gas discharge chambers or the pulsed energy to be supplied to thegas mixture in one or more gas discharge chambers) over time, it ispossible to see how the operating characteristic is changing over timeand to generally look at its instantaneous slope to see if thecharacteristic is rising rapidly in one direction such that it could beout of an acceptable range at a future time.

The measured characteristics can be averaged values of the measuredcharacteristics, for example, the average value of a measuredcharacteristic over a number (for example, hundreds or thousands) ofpulses of the energy supplied to the gas mixture 145. In this way,erroneous or outlier values can be excluded from the calculation toavoid imposing the restore gas maintenance scheme at times when it's notneeded.

The values of the measured characteristics can be averaged using anadaptive filter such as a recursive least squares (RLS) filter. Such afilter uses the average values of the measured characteristics tocalculate the instantaneous slope and offset value for a linear modelpredicting the time series and the value of the slope can therefore beused to determine the predictions. For example, if the control system160 predicts that one or more of the pulsed energy [that will berequired to be supplied to the gas mixture 145 in at least one of thegas discharge chambers 135, 335A, 335B] and the energy of the pulsedamplified light beam 120, 320, 370 [output from at least one of the gasdischarge chambers 135, 335A, 335B] will cross a pre-determinedrespective threshold in 10 million pulses (which is the future time),then the control system 160 can determine that the respective monitoredoperating characteristic will be out of an acceptable range at a futuretime, and the control system 160 assigns a fault state of 2 to thisdetermination. As another example, if the control system 160 predictsthat one or more of the pulsed energy [that will be required to besupplied to the gas mixture 145 in at least one of the gas dischargechambers 135, 335A, 335B] and the energy of the pulsed amplified lightbeam 120, 320, 370 [output from at least one of the gas dischargechambers 135, 335A, 335B] will cross a pre-determined respectivethreshold in 20 million pulses (which is the future time), then thecontrol system 160 can determine that the respective monitored operatingcharacteristic is at risk for being out of an acceptable range at afuture time, and the control system 160 assigns a fault state of 1 tothis determination. If the control system 160 assigns the fault state of1 to a determination, then the control system 160 could select therestore gas maintenance scheme that applies a standard gas injectionscheme to the gas discharge system 105, 305 (and thereby restore the gasinjection levels to their baseline value, the value before the risk tookplace). If the control system assigns the fault state of 2 to adetermination, then the control system 160 could include a refill schemein addition to the restore gas maintenance scheme because the risk wouldbe much higher for a fault and a more immediate and aggressive set ofactions should be taken.

The adaptive filter may need to be reset when its linear model does notfit well with the prediction, which can happen when there is a change inoperating conditions of the gas discharge light source 100, 300. Theadaptive filter may also include a forgetting factor, and all variablescan be initialized upon refill because the slope may be limited inmagnitude and have a finite memory (and therefore cannot change veryfast).

The control system 160 can monitor the one or more operatingcharacteristics of the gas discharge light source 100, 300 bycalculating values of the operating characteristics. The control system160 can determine whether any of the one or more monitored operatingcharacteristics will be out of the acceptable range at a future time bydetermining whether any of the calculated values of the operatingcharacteristics will be out of the acceptable range at a future time.The control system 160 can calculate values of the operatingcharacteristics by calculating average values of the operatingcharacteristics.

The relative amount of the component gas in the gas mixture 145 of thegas discharge chamber 135, 335A, 335B can be increased by applying arefill scheme to the gas discharge chamber 135, 335A, 335B. The refillscheme includes: purging the gas mixture 145 from the gas dischargechamber 135, 335A, 335B, and filling the gas discharge chamber 135,335A, 335B with a fresh gas mixture that includes the component gas.

The control system 160 can determine whether any of the one or moremonitored operating characteristics will be out of the acceptable rangeat a future time by determining whether any of the one or more monitoredoperating characteristics is likely to be out of the acceptable range ata future time.

The control system 160 can determine whether any of the one or moremonitored operating characteristics will be out of the acceptable rangeat a future time by, for example, determining a rate of change of eachof the one or more monitored operating characteristics; and/ordetermining whether the rate of change for each of the one or moremonitored operating characteristics indicates whether that monitoredoperating characteristic is likely to be out of the acceptable range ata future time.

The control system 160 can also determine whether any of the one or moremonitored operating characteristics will be out of another acceptablerange at a future time. Moreover, if the control system 160 determinesthat any of the one or more monitored operating characteristics will beout of the other acceptable range at a future time, then the controlsystem 160 can send a signal to the gas supply system 150, 350 to applya refill scheme to at least one gas discharge chamber 135, 335A, 335B.The refill scheme includes purging the gas mixture 145 from the gasdischarge chamber 135, 335A, 335B, and filling the purged gas dischargechamber with fresh gas mixture 145 that includes the component gas.

Referring to FIG. 6, exemplary procedures 640, 645, 650 are performed bythe control system 160 for analyzing the operating characteristics(540), based on that analysis, determining whether a monitored operatingcharacteristic will be out of an acceptable range at a future time(545); and selecting a restore gas maintenance scheme (550).

During the procedure 640, the control system performs an analysis(640-1, 640-2, . . . 640-N) for each of the operating characteristicsthat are being monitored (535). The outputs from those respectiveanalyses (640-1, 640-2, . . . 640-N) are directed to the procedure 645,where respective determinations (645-1, 645-2, . . . 645-N) areperformed based on those outputs. If the determination relating tooperating characteristic 1 finds that operating characteristic 1 will beout of range at a future time, then the control system 160 selects arestore gas maintenance scheme associated with that determination(650-1). Similarly, if the determination relating to operatingcharacteristic 2 finds that operating characteristic 2 will be out ofrange at a future time, then the control system 160 selects a restoregas maintenance scheme associated with that determination (650-2). Thecontrol system 160 then reviews each of the selected restore gasmaintenance schemes 1, 2, . . . N for each operating characteristic andselects a final restore gas maintenance scheme for output. Thus, forexample, it is possible that the control system 160 determines that oneof the operating characteristics will be outside a range at a firstfuture time and that all remaining operating characteristics will beoutside the range at a second future time that is greater than the firstfuture time. In this case, the control system 160 can select a restoregas maintenance scheme for the entire light source 100 that requires afull refill of all of the gas mixtures in all of the chambers of the gasdischarge system 105, 305.

Referring to FIG. 7, exemplary procedures 740-1, 745-1, 750-1 aresequentially performed for the first operating characteristic. Theseexemplary procedures 740-1, 745-1, 750-1 can be applied to the otheroperating characteristics.

The procedure 740-1 generally determines how the operatingcharacteristic OC changes with the usage (UM) of the light source 100.The usage of the light source (UM) is measured in any useful unit andcan be measured in time (for example, seconds, minutes, hours, etc.)since the light source chamber 135 was last refilled. In otherimplementations, usage is measured by counting a number of pulses orshots of the pulsed amplified light beam 120 from a point in time atwhich the light source chamber 135 was last refilled.

Moreover, the procedure 740-1 also generally operates by averaging thevalues of the operating characteristic OC and usage UM in order tomonitor long term trends and discard changes in values that occur on ashort timescale.

Initially in the procedure 740-1, the control system 160 performs apre-processing averaging of the operating characteristic OC over thelight source usage UM (that can be measured in average number of shotsor pulses) (700-1). Averaging ensures that long term trends aremonitored. Next, the control system 160 determines whether the averagedoperating characteristic is a value outside of an acceptable range(705-1) in order to cull those values that are transient in nature fromthe next model calculation.

Assuming that the next value has not been culled (705-1), the controlsystem 160 updates a model of a fit between the usage UM and theoperating characteristic OC (710-1) with the averaged values of theoperating characteristic OC and the usage UM. It should be noted thatfollowing a refill of the gas mixture in the gas discharge chamber 135of the light source 100, the model can be set to a standard model tostart, and then with each new averaged value, the model is updated in arecursive manner (using a recursive least squares process). In someimplementations, the model can be a linear fit between the usage UM andthe operating characteristic OC. In this case, the control system 160outputs at 710-1 a value of the local slope (a measure of how theoperating characteristic OC changes over a certain usage UM) and theoffset in the operating characteristic OC. In other implementations, themodel is a non-linear fit between the usage UM and the operatingcharacteristic OC and the control system 160 outputs at 710-1 a set ofcoefficients of the model.

The control system 160 next predicts a value of the operatingcharacteristic OC for a first elapsed usage UM (715-1) based on theoutput from the updated model (710-1) and a value of the operatingcharacteristic OC for a second elapsed usage UM (720-1) based on theoutput from the updated model (710-1). For example, if the model is alinear fit, and the output of the updated model (710-1) is a local slopethat represents how the operating characteristic OC changes with usageUM, then the prediction can measure the operating characteristic OC thatcorresponds to the first elapsed usage UM (715-1) and the operatingcharacteristic OC that corresponds to the second elapsed usage UM(720-1). For example, the value of the operating characteristic OC forthe first elapsed usage UM (715-1) can be a value of the operatingcharacteristic OC after another 10 million pulses have elapsed (thisvalue of the operating characteristic OC can be referred to as the failOC). As another example, the value of the operating characteristic OCfor the first elapsed usage UM (715-1) can be a value of the operatingcharacteristic OC after another 20 million pulses have elapsed (thisvalue of the operating characteristic OC can be referred to as the riskOC).

Once these predicted values are calculated at 715-1, 720-1, then adetermination is made as to whether each predicted value exceeds apre-determined threshold or thresholds (730-1, 735-1, respectively).

Referring to FIG. 8, graphs 800, 805, 810 of the various outputs duringthe procedure 740-1 are shown. Graph 800 shows the value of the localslope (the change in operating characteristic OC versus elapsed usageUM) in the updated model at 710-1 versus the elapsed usage UM. Graph 805shows the predicted value of the operating characteristic OC after thesecond elapsed usage UM output at 720-1. Graph 810 shows the predictedvalue of the operating characteristic OC after the first elapsed usageUM output at 715-1. The upper threshold level 815 is shown because theoperating characteristic OC is trending upward in this example. If therisk signal 805 crosses the upper threshold level 815, then the controlsystem 160 deems that there is a risk that a fault will occur and afirst warning is issued. If the risk signal 805 crosses the upperthreshold level 815 and the fail signal 810 crosses the upper thresholdlevel 815, then the control system 160 deems that there is a fault isimminent and a second warning is issued. The control system 160 selectsthe appropriate restore gas maintenance scheme depending on the warningthat is issued.

As an example, if the operating characteristic OC that is beingmonitored (535) is the pulsed energy required to be supplied to the gasmixture 145 in a gas discharge chamber, then this can be monitored bymonitoring the voltage supplied to the energy source 140. The upperthreshold level 815 can be 1150 volts and the lower threshold level (notshown in FIG. 8) can be 870 volts. As another example, if the operatingcharacteristic OC that is being monitored (535) is the output energy ofthe pulsed amplified light beam output from at least one of the gasdischarge chambers, then the output energy of the pulsed amplified lightbeam can be monitored directly. The upper threshold level 815 can be 4millijoules (mJ) and the lower threshold level can be 0.2 mJ.

Referring to FIG. 4, in general, the control system 160 includes one ormore of digital electronic circuitry, computer hardware, firmware, andsoftware. The control system 160 includes memory 400, which can beread-only memory and/or random access memory. Storage devices suitablefor tangibly embodying computer program instructions and data includeall forms of non-volatile memory, including, by way of example,semiconductor memory devices, such as EPROM, EEPROM, and flash memorydevices; magnetic disks such as internal hard disks and removable disks;magneto-optical disks; and CD-ROM disks. The control system 160 can alsoinclude one or more input devices 405 (such as a keyboard, touch screen,microphone, mouse, hand-held input device, etc.) and one or more outputdevices 410 (such as a speaker or a monitor).

The control system 160 includes one or more programmable processors 415,and one or more computer program products 420 tangibly embodied in amachine-readable storage device for execution by a programmableprocessor (such as the processors 415). The one or more programmableprocessors 415 can each execute a program of instructions to performdesired functions by operating on input data and generating appropriateoutput. Generally, the processor 415 receives instructions and data frommemory 400. Any of the foregoing may be supplemented by, or incorporatedin, specially designed ASICs (application-specific integrated circuits).The control system 160 includes, for example, various processing systemssuch as a system 425 for receiving the data from the monitoring system155, a system 430 for analyzing this data and deciding what sort ofaction should occur, and a system 435 for outputting a signal to the gassupply system 150 based on the decision output from the system 430. Eachof these processing systems can be a set of computer program productsexecuted by one or more processors such as the processors. The controlsystem 160 can include other processing systems (represented genericallyas box 440) for performing other tasks not related to gas maintenance.

Other implementations are within the scope of the following claims.

For example, the one or more properties of the gas discharge system 105that can be determined by the control system 160 can include a propertyof the pulsed light beam 120 output from the gas discharge system 105.Exemplary properties that can be determined include an energy of thepulsed light beam 120, an optical divergence of the pulsed light beam120, and a bandwidth of the pulsed light beam 120. As another example,the one or more properties of the gas discharge system 105 that can bedetermined by the control system 160 can include an efficiency at whichthe gas discharge system 105 operates, the efficiency can be determinedby measuring various aspects of the pulsed light beam 120 as well as theenergy that is supplied to the energy source 140.

1. A method of operating a gas discharge light source comprising a gasdischarge system that includes one or more gas discharge chambers, eachgas discharge chamber housing an energy source, the method comprising:filling each of the gas discharge chambers in the gas discharge systemwith a respective gas mixture; for each gas discharge chamber, supplyinga pulsed energy to the respective gas mixture by activating its energysource to thereby produce a pulsed amplified light beam from the gasdischarge chamber; determining one or more properties of the gasdischarge system; selecting a gas maintenance scheme from among aplurality of possible schemes based on the determined one or moreproperties of the gas discharge system; and applying the selected gasmaintenance scheme to the gas discharge system; wherein a gasmaintenance scheme includes one or more parameters related to adding oneor more supplemental gas mixtures to the gas discharge chambers of thegas discharge system.
 2. The method of claim 1, wherein determining oneor more properties of the gas discharge system comprises determining oneor more properties of each of the gas discharge chambers in the gasdischarge system.
 3. The method of claim 1, wherein applying theselected gas maintenance scheme to the gas discharge system comprisesapplying the selected gas maintenance scheme to each of the gasdischarge chambers of the gas discharge system.
 4. The method of claim1, wherein the gas discharge system includes two gas discharge chambers.5. The method of claim 1, wherein filling a gas discharge chamber withthe respective gas mixture comprises filling the gas discharge chamberwith a mixture of a gain medium and a buffer gas.
 6. The method of claim5, wherein filling a gas discharge chamber with the mixture of the gainmedium and the buffer gas comprises filling the gas discharge chamberwith a gain medium that includes a noble gas and a halogen, and a buffergas that includes an inert gas.
 7. The method of claim 6, wherein thenoble gas includes argon, krypton, or xenon; the halogen includesfluorine; and the inert gas includes helium or neon.
 8. The method ofclaim 1, wherein supplying the pulsed energy to a respective gas mixtureby activating its energy source comprises applying a pulsed voltage to apair of electrodes within the gas discharge chamber so that anelectrical stimulation is applied to a halogen within the gas mixture.9. The method of claim 1, wherein determining one or more properties ofthe gas discharge system comprises determining an age of at least one ofthe gas discharge chambers of the gas discharge system based on one ormore of: how many times that gas discharge chamber has been filled withthe gas mixture and how often the energy source of that gas dischargechamber has been activated.
 10. The method of claim 1, whereindetermining the one or more properties of the gas discharge system,selecting the gas maintenance scheme, and applying the selected gasmaintenance scheme to the gas discharge system occur while pulsed energyis being supplied to the gas mixture of the one or more gas dischargechambers.
 11. The method of claim 1, wherein determining the one or moreproperties of the gas discharge system, selecting the gas maintenancescheme, and applying the selected gas maintenance scheme to the gasdischarge system occur while pulsed energy is not being supplied to thegas mixture of any of the gas discharge chambers.
 12. The method ofclaim 1, wherein: selecting the gas maintenance scheme from among theplurality of possible gas maintenance schemes comprises selecting astandard gas injection scheme if it is determined that an age of atleast one of the gas discharge chambers is in a first range, andapplying the selected gas injection scheme to the gas discharge systemcomprises pumping at least a first amount of a buffer gas into the gasmixture of the at least one gas discharge chamber; and selecting the gasmaintenance scheme from among the plurality of possible gas maintenanceschemes comprises selecting a conservation gas injection scheme if it isdetermined that an age of the at least one gas discharge chamber is in asecond range, and applying the selected conservation gas injectionscheme to the gas discharge system comprises pumping a second amount ofthe buffer gas into the gas mixture of the at least one gas dischargechamber, the second amount being lower than the first amount.
 13. Themethod of claim 12, wherein selecting the gas maintenance scheme fromamong the plurality of possible gas maintenance schemes comprisesselecting another conservation gas injection scheme if it is determinedthat an age of the at least one gas discharge chamber is in a thirdrange, and applying the selected gas injection scheme to the gasdischarge system comprises pumping a third amount of the buffer gas intothe gas mixture of the at least one gas discharge chamber, the thirdamount being lower than the first amount but greater than the secondamount.
 14. The method of claim 13, wherein the first range is a valueless than or equal to a lower value, the second range is a value greaterthan the lower value, and the third range is a value that is greaterthan an upper value.
 15. The method of claim 13, wherein the first rangeis a first value of the age, the second range is a second value of theage, and the third range is a third value of the age.
 16. The method ofclaim 12, wherein the first range is a first value of the age and thesecond range is a second value of the age.
 17. The method of claim 12,wherein the second range is distinct from the first range.
 18. Themethod of claim 1, wherein: selecting the gas maintenance scheme fromamong the plurality of possible gas maintenance schemes comprisesselecting a standard gas injection scheme if it is determined that anage of at least one of the gas discharge chambers is in a first range,and applying the selected gas injection scheme to the gas dischargesystem comprises performing an injection of a buffer gas into the gasmixture of the at least one gas discharge chamber at a first temporalfrequency; and selecting the gas maintenance scheme from among theplurality of possible gas maintenance schemes comprises selecting aconservation gas injection scheme if it is determined that the age ofthe at least one gas discharge chamber is in a second range, andapplying the selected conservation gas injection scheme to the gasdischarge system comprises performing an injection of the buffer gasinto the gas mixture of the at least one gas discharge chamber at asecond temporal frequency that is different from the first temporalfrequency.
 19. The method of claim 18, wherein the second temporalfrequency is less than the first temporal frequency.
 20. The method ofclaim 18, wherein performing an injection of the buffer gas into the gasmixture of the at least one gas discharge chamber also includesinjecting one or more components of the gain medium into the gas mixtureof the at least one gas discharge chamber.
 21. The method of claim 1,further comprising: monitoring one or more operating characteristics ofthe gas discharge light source; determining whether any of the one ormore monitored operating characteristics will be out of an acceptablerange at a future time; and if it is determined that any of the one ormore monitored operating characteristics will be out of an acceptablerange at a future time, then: selecting a restore gas maintenance schemeand applying the selected restore gas maintenance scheme to the gasdischarge system.
 22. The method of claim 21, wherein applying theselected restore gas maintenance scheme to the gas discharge systemcomprises applying one or more of a restore injection scheme and arefill scheme to the gas discharge system.
 23. The method of claim 22,wherein applying the selected restore injection scheme to the gasdischarge system comprises performing an injection of a buffer gas intothe gas mixture of at least one of the gas discharge chambers thatincreases a relative amount of the buffer gas in the gas mixture of theat least one gas discharge chamber.
 24. The method of claim 23, whereinperforming the injection of the buffer gas into the gas mixture of theat least one gas discharge chamber that increases the relative amount ofthe buffer gas in the gas mixture of the at least one gas dischargechamber comprises one or more of altering a temporal frequency at whichthe injection is performed and pumping a different amount of buffer gasinto the gas mixture than was pumped before it was determined that anyof the one or more monitored operating characteristics will be out of anacceptable range.
 25. The method of claim 24, wherein: altering thetemporal frequency at which the injection is performed comprisesincreasing a frequency at which the injection is performed; and pumpingthe different amount of buffer gas into the gas mixture than was pumpedbefore it was determined that any of the one or more monitored operatingcharacteristics will be out of an acceptable range comprises pumpingless buffer gas into the gas mixture than was pumped before it wasdetermined that any of the one or more monitored operatingcharacteristics will be out of an acceptable range.
 26. The method ofclaim 1, wherein applying the selected gas maintenance scheme to the gasdischarge system occurs after each of the gas discharge chambers isfilled with its respective gas mixture.
 27. The method of claim 26,further comprising, after the selected gas maintenance scheme is appliedto the gas discharge system, performing a refill scheme to the gasdischarge chamber, the refill scheme comprising: emptying each of thegas discharge chambers of the gas discharge system; and refilling eachgas discharge chamber with fresh gas mixture.
 28. The method of claim 1,wherein a gas maintenance scheme includes one or more of the followingparameters: an amount of a component gas that is to be pumped into thegas mixture; and a temporal frequency at which an injection of thecomponent gas into the gas mixture is performed.
 29. The method of claim28, wherein the component gas is a buffer gas of the gas mixture, thegas mixture includes a gain medium.
 30. The method of claim 1, whereinone of the gas maintenance schemes is a refill scheme that includesemptying at least one of the gas discharge chambers and refilling thatemptied gas discharge chamber with fresh gas mixture.
 31. A method ofoperating a gas discharge light source that includes one or more gasdischarge chambers, the method comprising: filling a gas dischargechamber with a gas mixture, the gas discharge chamber housing an energysource; supplying a pulsed energy to the gas mixture by activating theenergy source to thereby produce a pulsed amplified light beam;determining one or more properties of the gas discharge chamber;selecting an injection scheme from among a plurality of possibleinjection schemes based on the determined one or more properties of thegas discharge chamber; and applying the selected injection scheme to thegas discharge chamber; wherein an injection scheme includes one or moreparameters related to adding one or more supplemental gas mixtures tothe gas discharge chamber.
 32. A gas discharge light source comprising:a gas discharge system that includes one or more gas discharge chambers,each gas discharge chamber housing an energy source and containing a gasmixture that includes a gain medium; and a gas maintenance systemcomprising: a gas supply system; a monitoring system; and a controlsystem coupled to the gas supply system and the monitoring system andconfigured to: provide a signal to activate each energy source tothereby produce a pulsed amplified light beam from its gas dischargechamber; receive information from the monitoring system and determineone or more properties of the gas discharge system based on thisreceived information; select a gas maintenance scheme from among aplurality of possible schemes based on the determined one or moreproperties of the gas discharge system; and provide a signal to the gassupply system to thereby apply the selected gas maintenance scheme tothe gas discharge system; wherein a gas maintenance scheme includes oneor more parameters related to adding one or more supplemental gasmixtures to the gas discharge chambers of the gas discharge system. 33.The light source of claim 32, wherein the component gas includes abuffer gas and the gas mixture contained within the gas dischargechamber also includes a buffer gas.
 34. The light source of claim 33,wherein: the gain medium includes a noble gas and a halogen; the buffergas includes an inert gas; and the inert gas of the component gasincludes an inert gas.
 35. The light source of claim 32, wherein the gasdischarge system includes a master oscillator having a master oscillatorgas discharge chamber providing a seed light beam, and a power amplifierhaving a power amplifier gas discharge chamber that receives the seedlight beam, wherein the one or more gas discharge chambers include themaster oscillator gas discharge chamber and the power amplifier gasdischarge chamber.
 36. The light source of claim 32, wherein the gassupply system includes one or more gas sources and a valve systemfluidly connected to both the one or more gas sources and the one ormore gas discharge chambers.
 37. The light source of claim 36, whereinthe one or more gas sources include: a tri-mix gas source includingthree gases, wherein the three gases include one or more of: halogenfluorine, a noble gas, and a rare gas; and a bi-mix gas source includingtwo gases, wherein the two gases include one or more of a noble gas andanother gas and lack any fluorine.